A large group of retroviruses, which induce malignancies, do not have appreciated oncogenes in their genomes. Tumor induction by these viruses is explained by proviral integration in the proximity of a cellular proto-oncogene followed by activation of its expression. This scenario, however, may have to be reconsidered, because we have identified a group of exogenous mouse mammary tumor viruses (MMTV) that fail to induce mammary tumors in susceptible mice, even though they are all replication-competent, produced at normal titers and can activate proto-oncogene expression. The C3H/HeJ and C3H/HeN substrains diverged more than 50 years ago from CSH/St, an MMTV (C3H)-infected, and mammary tumor- prone progenitor strains. We found that C3H/HeJ mice carry MMTV variants that are all recombinants between the original wild-type MMTV (C3H), still carried by the C3H/HeN strain, and endogenous Mtv1 present in both C3H substrains. These recombinant viruses have completely lost the ability to cause mammary tumors in all C3H mice, while wild-type MMTV (C3H) is highly tumorigenic in mice from either strain. Clearly, loss of tumor-inducing capacity is based on the alteration of the viral genome. Using a panel of chimeric viruses, the tumor-attenuating sequences have been mapped to the capsid (CA) domain of the gag gene. Importantly, we found that in contrast to C3H/He mice, mice of the BALB/cJ stain are highly susceptible to mammary tumors caused by all MMTV variants, enabling analysis of the host genes involved in interaction with Gag. We have established that a single gene tentatively called "mammary tumor susceptibility" or mts, which maps to Chromosome 14, controls Gag-dependent tumor susceptibility. We hypothesize that in addition to the well-known mechanism of MMTV-induced tumorigenesis, i.e. insertional mutagenesis, MMTV gag-encoded proteins also directly contribute to the transformation of mammary epithelial cells by cooperating with cellular protein Mts (directly or indirectly) to increase the oncogenic potential of the virus. The experimental plan described in this proposal will allow us to determine the mechanism by which Gag contributes to mammary tumorigenesis. Relevance to public health: The proposed studies will provide fundamental insights into signal transduction pathway involved in mammary gland tumorigenesis and ultimately help to identify potential targets for anti-tumor chemotherapy.